Where it is desired to convert mechanical energy into electrical energy, a dynamoelectric machine in the form of a generator is typically utilized. In principle, such machines achieve the desired energy conversion by utilizing a phenomenon known as magnetic induction whereby relative motion between a magnetic field and an electrical conductor disposed within the magnetic field will induce a flow of current in the electrical conductor. In practice, such machines typically employ a rotating member known as a rotor mounted within a stationary member known as a stator, with the rotor being rotatably driven by some form of prime mover such as a turbine engine operably connected to the rotor.
In general, it is desirable to design such a machine such that it is not necessary to utilize devices such as brushes or slip rings for conducting electrical current to or from the rotor because such devices are subject to wear and therefore reduce the reliability of the generator.
Where the need for electrical current is relatively small, such a "brushless" design is conveniently provided by placing the electrical conductor in the stator and providing one or more permanent magnets within the rotor to create a magnetic field. As the rotor is driven, the rotating magnetic field induces a flow of current in the stationary electrical conductor which may be conveniently connected to deliver the induced current to an electrical load connected to the stationary electrical conductor.
Where the need for electrical current is substantial, however, other means for producing an electrical field in the rotor are typically utilized. Such means are generally provided by placing a winding in the rotor which when connected to a source of current becomes, in effect, an electromagnet capable of producing a rotating magnetic field of sufficient intensity to allow generation of the desired substantial amount of electrical current. Some means for supplying the electrical current to "excite" the winding in the rotor, which is generally known as a main field winding, must be provided, however, preferably without resorting to the use of slip rings or brushes.
It is known that the required excitation current may be conveniently supplied without resorting to brushes or slip rings in a generator configuration typically known as a brushless alternator which utilizes magnetic induction to produce and to couple the excitation current to the main field winding.
A typical brushless alternator has three distinct generating systems, including a main generator, an exciter generator, and a permanent magnet generator (PMG). Each of the generating systems include a rotating member mounted within a common rotor of the brushless generator and a stationary member mounted within a common stator assembly of the brushless alternator. The rotating member of the permanent magnet generator includes one or more permanent magnets, known as a PMG field assembly, which establishes a rotating magnetic field when the rotor is driven by a prime mover. The rotating magnetic field established by the permanent magnets is employed to induce an alternating current (AC) in a stationary PMG armature winding which is located within the stator of the brushless alternator. This induced alternating current is in turn rectified in a stationary rectifier connected to the stator to provide a direct current (DC) which is supplied to a stationary field winding of the exciter generator also located within the stator. The direct current is employed by the field winding of the exciter generator to produce a stationary magnetic field within which an exciter armature winding attached to the rotor is rotated to generate a higher level of current, typically in the form of a three-phase alternating current which is connected, by means to be described hereinafter, to a main field winding located within the rotor. The main field winding generates a rotating magnetic field which is used to induce the main flow of AC current from the main generator stator to the electrical load.
In order to generate a magnetic field of sufficient strength in the main field winding, it is necessary to utilize a direct current as opposed to an alternating current. Since the output of the exciter armature winding is an alternating current, some means must be provided within the rotor for rectifying this alternating current to a direct current. A rotating rectifier assembly located within the rotor is utilized for this purpose.
Rotating rectifier assemblies carried in a rotor of a generator are subject to high centrifugal loading, particularly in generators which run at high speeds, requiring that care be taken to ensure that the rectifier has sufficient structural integrity or is adequately supported within the rotor to withstand such loading. Diode semiconductor devices typically used in the fabrication of such rotating rectifier assemblies also dissipate power during use in the form of heat. Without proper cooling, the diode semiconductors will fail.
It is also common practice to provide an electrical noise suppression resistor within the rotor of the brushless alternator, with the suppression resistor being connected in a parallel circuit relationship to the main field winding of the main generator. These resistors also dissipate heat and are subject to the same structural and thermal environment as the rectifier.
In addition to concerns regarding the structural integrity and cooling of the rectifiers, the compactness or lack thereof of the rotor, and particularly the axial length of the rotor, will dictate certain bearing and housing size selections and thus greatly affect the weight and size of the brushless generator. Since the rotating rectifier forms part of the rotor, and therefore influences the size of the rotor, it has long been a goal of generator designers to reduce the rotating rectifier to a simple, elemental form having minimum size and weight.
Through the years, a number of approaches to solving the above problems have been utilized.
In one approach, compression-bonded, discoidal-shaped diode wafers are packaged individually in a housing including means such as a Belleville washer for compressively loading the diode wafers against electrical connection means disposed within the individual housings. A plurality of such diodes, each within its individual housing, is then mounted directly within the rotor or on structural support plates carried by the rotor. Electrical connections required to join the individual diodes into a rectifier circuit are provided by wires or other nonstructural conductor means which are brazed, soldered, crimped, or otherwise mechanically attached to the individual diode packages. Rotating rectifiers utilizing this approach have been shown to work well but are rather bulky due to the multiple housings, springs, and electrical connection means, etc., which are required when each diode wafer is individually packaged in its own housing. U.S. Pat. No. 4,482,827 to Baldwin is exemplary of a rotating rectifier utilizing this approach.
In a second approach, which eliminates much of the redundant packaging inherent in the first approach, the compression-bonded diode wafers are removed from their individual packages and mounted within a common housing together with means for providing the compressive force necessary to ensure good electrical and thermal contact of the diode wafers with electrical connections and heat sinks which are also provided within the common housing. U.S. Pat. Nos. 4,570,094 to Trommer, 4,581,695 to Hoppe, 4,603,344 to Trommer, 4,628,219 to Troscinski, 4,806,814 to Nold, 5,003,209 to Huss et al, are exemplary of rotating rectifiers utilizing the second approach.
Advantages of this approach as compared to Baldwin are that a smaller, more space-efficient package may typically be provided due to elimination of redundant packaging elements such as the individual housing and compressive loading means. Rotating rectifiers utilizing this approach have historically been provided in the form of a modular package which may also include a suppression resistor. These modular packages typically utilize some combination of screw terminals or axially-extending connector pins at one or both ends which provide an additional advantage by facilitating installation and repair of the rectifier and suppression resistor.
In a third approach, as illustrated in U.S. Pat. Nos. 4,959,707 to Pinchott and 4,987,328 to Shahamat, the means for providing compressive force to the diode wafers is eliminated by brazing or soldering the diodes to plates which are then supported within an electrically nonconductive housing and interconnected by electrically conductive means within a common housing. With the third approach, as in the second approach, a resistor may be included within the housing and electrical attachment means in the form of screw terminals, and pins may be utilized to provide a modular package in order to facilitate installation and repair of the rectifier and resistor assembly. As with the first and second approaches, a structural housing is generally required with this approach to maintain the spatial relationship between the internal components and provide structural support for the internal components which are typically not utilized in a structural capacity.